1. Technical Field
The present invention relates to local oscillators and, in particular, to phase locked loop and direct conversion circuits using a local oscillator.
2. Description of Related Art
The increased demand in the wireless phone industry for highly integrated small form factor transmitter application specific integrated circuits (ASIC) requiring low external component count constrains the choice of possible implementation architectures. One very often used transmitter architecture that meets this requirement is based on direct conversion.
Direct conversion transmitters suffer from a phenomenon called Local Oscillator (LO) injection pulling. The oscillator in the frequency synthesizer operates at the same frequency as, or an integer multiple of, the transmitter output frequency. Due to limited reverse isolation, the transmitted output signal, or its harmonics, couple to the LO and seriously degrade its performance.
There are several ways to handle reverse modulation. One of the approaches to suppress reverse modulation is to improve printed circuit board (PCB) layout, decoupling and shielding. Although this approach seems to be simple, repeatability can be an issue. A fractional PLL based transmitter design is a second option. This solution improves local oscillator injection pulling, but does not completely eliminate the LO injection pulling effect. Another solution includes deployment of the “offset” LO scheme where the LO operates at frequency that is not harmonically related to a frequency that is intentionally radiated by transmitter. Because of this, the “offset” LO scheme is immune to injection pulling.
A particular problem with offset LO schemes concerns the presence of unwanted spurious response due to mixing. A need exists in the art for an offset LO scheme with improved spurious noise filtering.